Photodetectors of this type, such as PIN photodiodes, comprise a cathode (n-region), an insulation (i-region) and an anode (p-region). This structure is produced on a substrate, for example a silicon substrate, by means of a first implantation, epitaxy and a second implantation.
In order to achieve good light absorption in a desired wavelength range, simple ARC layers (anti-reflecting coating) are deposited on the photodiode, or the integrated photodetectors. The disadvantage of these layers, however, is that they are optimized for a specific wavelength and are thus unsuitable for multispectral applications.
It is possible within limits to achieve a broadened absorption maximum by realizing a corresponding ARC layer construction through multiply deposited thin dielectric layers. However, this method has particular disadvantages in combination with the photodiode integrated into a chip technology.
In particular, the realization of multiple layers means that process costs are multiplied correspondingly in comparison with a single layer deposition. Moreover, this increases the throughput time in production, thereby causing an obstructive build-up at the respective coating installations (bottleneck effect). Moreover, it is difficult to realize the required accuracy of the layers to layer thicknesses of a few nanometers.
Furthermore, in semiconductor fabrication, the required refractive indices of light cannot be realized or are not available. In the case of ARC layers, in particular in the case of multilayers, a certain attenuation of the incident light is unavoidable.
In order to compensate for, or to avoid, this disadvantage and to be able to absorb as much light as possible, it is known to form patterned micropyramids on the surface of the photodetector. By virtue of these micropyramids, the absorption behavior becomes independent of the wavelength of the light radiated in. As a result, the light is also predominantly absorbed in the upper part of the photodetector, which has the consequence that the required epitaxial thickness can be reduced during the production of the photodetector.
Examples of ARC layers and micropyramids are described in “Silicon Solar Cells, Advanced Principles and Practice,” Martin A. Green, published by the Centre for Photovoltaic Devices and Systems, University of New South Wales, Sydney, printed by Bridge Printery Pty. Ltd, March 1995.
These micropyramids on the surface of the photodetector may be realized by selective etching by uncovering the (111) crystal planes, thus giving rise to pyramids with square base areas. Furthermore, this document also describes the use of inverted pyramids (that is to say pyramids standing on their vertex) in order to improve the absorption of light in the photodetector. However, the production of inverted pyramids of this type is associated with a considerable complexity, so that this variant is not suitable for a practical application.